Internet Engineering Task Force SIMPLE WG
Internet Draft Rosenberg
draft-ietf-simple-winfo-package-00.txt dynamicsoft
July 13, 2001
Expires: January 2002
A SIP Event Sub-Package for Watcher Information
STATUS OF THIS MEMO
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all provisions of Section 10 of RFC2026.
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Abstract
document defines the watcher information sub-package for the SIP
event infrastructure. Watcher information refers to the set of users
subcribed to a particular resource within a particular event package.
This set changes dynamically as users subscribe, unsubscribe, are
approved, or are rejected. A subscriber can subscribe to this
information, and therefore learn about changes to it. This event
package is a sub-package because it can be applied to any event
package, including itself.
1 Introduction
The SIP event infrastructure is described in [1]. It defines a
generic framework for subscription to, and notification of, events
related to SIP systems. The framework defines the methods SUBSCRIBE
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and NOTIFY, and introduces the notion of a package. A package is a
concrete application of the event framework to a particular class of
events. Packages have been defined for user presence [2] and for
message waiting indications [3], for example.
This draft defines a "sub-package" within the SIP events framework. A
sub-package has all the properties of a regular SIP event package.
However, it is always associated with some other event package, and
can always be applied to any event package, including the sub-package
itself.
The sub-package defined here is for watcher information, and is
denoted with the token "winfo". For any event package, such as
presence, there is a set of users who are subscribed to a particular
user within that package. This set of users changes over time as new
subscriptions come, old subscriptions expire, and subscriptions are
approved or rejected. The set of users subscribed to a particular
event package, and the state of their subscriptions, is referred to
as watcher information. Since this state is itself dynamic, it is
reasonable to subscribe to it in order to learn about changes to it.
The watcher info event sub-package is meant to facilitate exactly
that - tracking the state of subscriptions to a user in another
package.
To denote this sub-package, the name is constructed by appending
".winfo" to the name of whatever package is being tracked. For
example, the set of people subscribed to presence is defined by the
"presence.winfo" package.
2 Usage Scenarios
There are many useful applications for the watcher information sub-
package.
2.1 Presence Authorization
The motivating application for this package is presence
authorization. When user A subscribes to the presence of user B, the
subscription needs to be authorized. Frequently, that authorization
needs to occur through direct user intervention. For that to happen,
B's software needs to become aware that a presence subscription has
been requested. This is supported through watcher information. B's
client software would SUBSCRIBE to the watcher information for the
presence of B:
SUBSCRIBE sip:B@foo.com SIP/2.0
Via: SIP/2.0/UDP pc34.foo.com
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From: sip:B@foo.com;tag=123
To: sip:B@foo.com
Call-ID: 9987@1.2.3.4
CSeq: 9887 SUBSCRIBE
Contact: sip:B@pc34.foo.com
Event: presence.winfo
The policy of the server is such that it allows B to subscribe to its
own watcher information. So, when A subscribes to B's presence, B
gets a notification of the change in watcher information state:
NOTIFY sip:B@pc34.foo.com SIP/2.0
Via: SIP/2.0/UDP pc34.foo.com
From: sip:B@foo.com;tag=xyz
To: sip:B@foo.com;tag=123
Call-ID: 9987@1.2.3.4
CSeq: 1288 NOTIFY
Contact: sip:B@server.foo.com
Event: presence.winfo
Content-Type: text/xml+winfo
Content-Length: ...
<watcherinfo>
<presentity uri="sip:B@foo.com">
<watcher uri="sip:A@foo.com" status="pending"/>
</presentity>
</watcherinfo>
This indicates to B that A has subscribed, and that the server is
awaiting approval. B's software can alert B that this subscription is
awaiting authorization. B can then go to a web page and set policy
for that subscriber.
2.2 Blacklist Alerts
Much benefit is derived by allowing applications to subscribe to
watcher information, in order to provide value-added features. An
example application is blacklist alerts. In this scenario, an
application maintains a list of known "bad guys". A user, Joe, signs
up for service with the application, presumably by going to a web
page and entering in there presence address. When someone attempts to
SUBSCRIBE to Joe's user presence, the application learns of this
subscription. It checks the watcher's URL against the database of
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known bad guys. If there is a match, it sends email to Joe letting
him know about this.
For this application to work, the application itself subscribes to
the presence.winfo for Joe. This allows it to be notified when
someone subscribes to Joe's presence, so that it can run the name
against its database.
For this application to work, Joe needs to make sure that the
application is allowed to subscribe to his presence.winfo.
3 Package Definition
This section fills in the template that is needed in order to fully
specify a SIP event package.
3.1 Package Name
As a sub-package, the watcher information sub-package has many names.
Watcher information for any package foo is denoted by the name
"foo.winfo". Recursive sub-packaging is explicitly allowed (and
useful), so that "foo.winfo.winfo" is a valid package name.
3.2 SUBSCRIBE Bodies
A SUBSCRIBE for a watcher information package MAY contain a body.
This body would serve the purpose of filtering the subscription. For
example, in the case of presence, the body might indicate that
notifications should contain full state every time something changes,
and that the time the subscription was first made should not be
included in the notifications.
A SUBSCRIBE for a watcher information package MAY be sent without a
body. This implies the default subscription filtering policy. The
default policy is:
o Notifications are generated every time there is any change in
the state of the watcher information.
o Notifications do not normally contain full state; rather, they
only indicate the watcher whose state has changed. Full state
(the list of all watchers) is sent in the NOTIFY triggered
from a SUBSCRIBE.
3.3 Expiration
Watcher information changes as users subscribe to a particular
package, or their subscriptions time out. As a result, the state of
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watcher information can change very dynamically, depending on the
number of subscribers for a particular user in a given package. The
rate at which subscriptions time out depends on how long a user
maintains its subscription.
As a result of these factors, it is difficult to define a broadly
useful default value for the lifetime of a subscription to watcher
information. We arbitrarily choose one hour. However, clients SHOULD
include in Expires header specifying their preferred duration.
3.4 NOTIFY Bodies
The body of the notification contains a watcher information document.
This document describes some or all of the set of watchers for a
given package, and the state of their subscriptions. All subscribers
MUST support the watcherinfo+xml format described in [4], and MUST
list its MIME type, application/watcherinfo+xml, in an Accept header
present in the SUBSCRIBE request.
Other watcher information formats might be defined in the future. In
that case, the subscriptions MAY indicate support for other presence
formats. However, they MUST always support and list
application/watcherinfo+xml as an allowed format.
Of course, the notifications generated by the server MUST be in one
of the formats specified in the Accept header in the SUBSCRIBE
request.
3.5 Authorization Considerations
The watcher information for a particular package contains sensitive
information. Therefore, all subscriptions SHOULD be authenticated and
then authorized before approval. Authorization policy is at the
discretion of the administrator, as always. However, a few
recommendations can be made.
It is RECOMMENDED that subscriptions to the watcher information for
some package foo for user A be allowed from user A themselves. That
is, it is RECOMMENDED that a user be allowed to subscribe to their
own watcher information. This is true recursively, so that we
recommend a user be able to subscribe to the watcher information for
their watcher information for any package.
It is RECOMMENDED that subscriptions to the watcher information for
some package foo for user A be allowed from some other user B if B is
an authorized subscriber to A within the package foo. However, it is
STRONGLY RECOMMENDED that the notifications sent to B only contain
the state of B's own subscription. In other words, it is RECOMMENDED
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that a user be allowed to monitor the state of their own
subscription.
To avoid infinite recursion, it is RECOMMENDED that only a user A be
allowed to subscribe to foo.winfo.winfo for any foo (such a
subscription from A is actually needed to approve the blacklisting
application). It is also RECOMMENDED that by default, a server does
not support any subscriptions to foo.winfo.winfo.winfo or any other
deeper sub-packages.
OPEN-ISSUE: Are we sure about these recommendations? Any
cases where this is really useful to allow others to
subscribe to foo.winfo.winfo?
3.6 Generation of Notifications
Notifications may be generated for watcher information on package
foo, when the subscription state for a user on package foo changes.
There is an underlying state machine, described below, which governs
the subscription state of a user in any package. Notifications are
generated on transitions in this state machine.
3.6.1 The Watcherinfo State Machine
The underlying state machine for a subscription is shown in Figure 1.
Initially, there is no state allocated for a subscriber (the init
state). When a subscription arrives, the subscription FSM is created.
The next state depends on whether policy exists for the subscription.
If there is an existing policy that determines that the subscription
is forbidden, it moves into the terminated state immediately, where
the FSM can be destroyed. If there is existing policy that determines
that the subscription is authorized, the FSM moves into the active
state. This state indicates that the subscriber will receive
notifications.
If, when a subscription arrives, there is no authorization policy in
existence, the subscription moves into the pending state. In this
state, the server is awaiting an authorization decision. No
notifications are generated, but the subscription FSM is maintained.
If the authorization decision comes back positive, the subscription
is approved, and moves into the active state. If the authorization is
negative, the subscription is rejected, and the FSM goes into the
terminated state. It is possible that the authorization decision can
take a very long time. In fact, no authorization decision may arrive
until after the subscription itself expires. If a pending
subscription expires, it moves into the waiting state. This state is
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more, the subscription is pending since no policy is defined for it.
This process could continue indefinitely. The waiting state ensures
that B can find out about this subscription attempt.
This state is also needed to allow for authorization of fetch
attempts.
Of course, policy may never be specified for the subscription. As a
result, the server can timeout the waiting subscription. The value
for this timeout is system dependent. It SHOULD be several times
larger than the default expiration time for the package being
watched.
If, while in the waiting state, the subscription is refreshed through
another SUBSCRIBE, it moves back into the pending state.
At any time, the server can deactivate a subscription. Deactivation
implies that the subscription is discarded without a change in
authorization policy. This may be done in order to trigger refreshes
of subscriptions for a graceful shutdown or subscription migration
operation.
3.6.2 Applying the state machine
The server MAY generate a notification to subscribers of watcher
information on a transition of the state machine. Whether it does or
does not is policy dependent. However, several guidelines are
defined.
Consider some event package foo. A subscribes to B for events within
that package. A also subscribes to foo.winfo for B. In this scenario
(where the subscriber to foo.winfo is also a subscriber to foo for
the same user), it is RECOMMENDED that A receive notifications only
about the changes in its own subscription. Furthermore, it is
RECOMMENDED that state machine transitions be reported only if A's
subscription to B for package foo was approved. That is, the first
notification that A should ever receive for foo.winfo is its
transition from pending to approved. If A's subscription to B for foo
is later rejected, it is RECOMMENDED that A not receive notifications
of this, or any other changes. All of these guidelines are to protect
the privacy of B; specifically, so that A cannot know about
rejections of its subscriptions to B for some particular package
unless explicitly permitted by B.
As a general rule, when a subscriber is authorized to receive
notifications about more than one watcher, it is RECOMMENDED that
notifications contain information about those watchers which have
changed state (and thus triggered a notification), instead of
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delivering the current state of every watcher in every notification.
However, notifications triggered as a result of a fetch operation (a
SUBSCRIBE with Expires of 0) SHOULD result in the full state of all
watchers (of course, only those watchers that have been authorized to
be divulged to the subscriber) to be present in the NOTIFY.
3.7 Rate Limitations on NOTIFY
For reasons of congestion control, it is important that the rate of
notifications not become excessive. As a result, it is RECOMMENDED
that the server not generate notifications for a single subscriber at
a rate faster than once every 5 seconds.
4 Example Usage
The following section discusses some sample applications and call
flows using the watcherinfo package.
4.1 Presence Authorization
In this example, a user Joe, sip:joe@bar.com provides presence
through the bar.com presence server. Joe subscribes to his own
watcher information, in order to learn about people who subscribe to
his presence, in order to approve them. Joe sends the following
SUBSCRIBE request:
SUBSCRIBE sip:joe@bar.com SIP/2.0
Via: SIP/2.0/UDP pc34.bar.com
From: sip:joe@bar.com;tag=123
To: sip:joe@bar.com
Call-ID: 9987@1.2.3.4
CSeq: 9887 SUBSCRIBE
Contact: sip:joe@pc34.bar.com
Event: presence.winfo
The server responds with a 401 to authenticate, and Joe resubmits the
SUBSCRIBE with credentials. The server then authroizes the
subscription, since it allows Joe to subscribe to his own watcher
information for presence. It responds with a 200 OK:
SIP/2.0 200 OK
Via: SIP/2.0/UDP pc34.bar.com;received=1.2.3.4
From: sip:joe@bar.com;tag=123
To: sip:joe@bar.com;tag=xyz
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